Electrochemical characterization of anti-corrosion coatings formed on 6061 aluminum alloy by plasma electrolytic oxidation in the corrosion inhibitor-enriched aqueous solutions

Bioactive Properties of Phosphate-Modified Silicon Oxycarbide Protective Coatings: Morphology and Functional Evaluation

This article presents a study on the functional properties and morphology of coatings based on amorphous silicon oxycarbide modified with phosphate ions and comodified with aluminum and boron. The objective of this modification was to enhance the biocompatibility and bioactivity without affecting its protective properties. The comodification was aimed toward stabilization of phosphate in the structure. The coatings were prepared according to the typical procedure for polymer-derived ceramics: synthesized via the sol-gel method, deposited using the dip-coating technique, and subsequently pyrolyzed. Comprehensive analyses of the morphology, surface properties, corrosion resistance, and bioactivity were conducted to assess their functional performance. The coatings exhibited uniform and smooth surfaces, with phase separation observed in the boron-modified SiBPOC series. Surface wettability and free energy measurements demonstrated that SiPOC and SiBPOC coatings possessed moderate hydrophilicity and favorable surface free energy for cell adhesion and bone tissue mineralization. Corrosion resistance tests in Ringer's solution revealed that SiBPOC coatings provided the highest protection against ion leaching, while SiAlPOC showed decreased resistance due to surface cracks. Bioactivity tests indicated calcium phosphate precipitation on the surface of all samples with higher hydroxyapatite formation on SiPOC and SiAlPOC coatings. In vitro tests using MG-63 osteoblast-like cells confirmed the biocompatibility of the coatings, with SiPOC and SiBPOC exhibiting the best combination of bioactivity, cell adhesion, and proliferation. These findings suggest that the phosphate- and boron-modified SiOC-based coatings are promising candidates for enhancing bone integration in orthopedic implants.

Study of amorphous alumina coatings for next-generation nuclear reactors: High-temperature in-situ and post-mortem Raman spectroscopy and X-ray diffraction

The present work focuses on the investigation of the thermal stability and structural integrity of amorphous alumina coatings intended for use as protective coatings on cladding tubes in Generation IV nuclear reactors, specifically in the Lead-cooled Fast Reactor (LFR) type. High-temperature Raman spectroscopy and high-temperature X-ray diffraction analyses were carried out up to 1050 °C on a 5 µm coating deposited by the pulsed laser deposition (PLD) technique on a 316L steel substrate. The experiments involved the in-situ examination of structural changes in the material under increasing temperature, along with ex-situ Raman imaging of the surface and cross-section of the coating after thermal treatments of different lengths. As it was expected, the presence of α-alumina was detected with the addition of other polymorphs, γ- and θ-Al2O3, found in the material after longer high-temperature exposure. The use of two structural analysis methods and two lasers excitation wavelengths with Raman spectroscopy allowed us to detect all the mentioned phases despite different mode activity. Alumina analysis was based on the emission spectra, while substrate oxidation products were identified through the structural bands. The experiments depicted a dependence of the phase composition of oxidation products and alumina's degree of crystallization on the length of the treatment. Nevertheless, the observed structural changes did not occur rapidly, and the coating's integrity remained intact. Moreover, oxidation signs occurred locally at temperatures exceeding the LFR reactor's working temperature, confirming the material's great potential as a protective coating in the operational conditions of LFR nuclear reactors.

The Effect of Sodium Tetrafluoroborate on the Properties of Conversion Coatings Formed on the AZ91D Magnesium Alloy by Plasma Electrolytic Oxidation

Magnesium and its alloys are widely used in many areas because of their light weight, excellent dimensional stability, and high strength-to-weight ratio. However, the material exhibits poor wear and corrosion resistance, which limits its use. Plasma electrolytic oxidation (PEO) is an effective surface modification method for producing ceramic oxide layers on Mg and their alloys. The influence of the additions of sodium tetrafluoroborate (NaBF4) and sodium fluoride (NaF) into alkaline-silicate electrolyte on the properties of the conversion layers formed in the magnesium AZ91D alloy has been investigated. Surface morphology and chemical composition were determined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The anticorrosive properties of the layers were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods in simulated body fluid (SBF). The presence of NaBF4 or NaF in the electrolyte increases the corrosion resistance of the protective layer. However, the best anticorrosive properties show the layers obtained in the presence of NaBF4. This is probably caused by the incorporation of boron and fluorine in the form of Mg (BF4)2 mainly in the barrier layer.